Purpose: Two-photon vision relies on the perception of pulsed infrared light due to two-photon absorption in visual pigments. This study aimed to measure human pupil reaction caused by a two-photon 1040-nm stimulus and compare it with pupil responses elicited by 520-nm stimuli of similar color.
Methods: Pupillary light reflex (PLR) was induced on 14 dark-adapted healthy subjects. Three types of fovea-centered stimuli of 3.5° diameter were tested: spirals formed by fast scanning 1040-nm (infrared [IR] laser) or 520-nm (visible [VIS] laser) laser beams and uniformly filled circle created by 520-nm LED (VIS light-emitting diode [LED]). The power of visible stimuli was determined with a dedicated procedure to obtain the same perceived brightness equivalent as for 800 µW used for two-photon stimulation.
Results: The minimum pupil diameter for IR laser was 88% ± 10% of baseline, significantly larger than for both VIS stimuli: 74% ± 10% (laser) and 69% ± 9% (LED). Mean constriction velocity and time to maximum constriction had significantly smaller values for IR than for both VIS stimuli. Latency times were similar for IR and VIS lasers and slightly smaller for VIS LED.
Conclusions: The two-photon stimulus caused a considerably weaker pupil reaction than one-photon stimuli of the same shape, brightness, and similar color. The smaller pupil response may be due to weaker two-photon stimulation of rods relative to cones as previously observed for two-photon vision. Contrary to normal vision, in a two-photon process the stray light is not perceived, which might reduce the number of stimulated photoreceptors and further weaken the PLR.